## **Core Concept**
At high altitudes, the atmospheric pressure decreases, which leads to a decrease in the partial pressure of oxygen (PO2) in the inspired air. The inspired PO2 can be calculated using the formula: Inspired PO2 = (Atmospheric Pressure - Water Vapor Pressure) * FiO2, where FiO2 is approximately 0.21 at sea level.

## **Why the Correct Answer is Right**
To calculate the inspired PO2 at an altitude of 6500 meters, we first need to convert the given atmospheric pressure from mmHg to a usable form and apply the formula. Given atmospheric pressure is 347 mmHg. The water vapor pressure at 37°C (body temperature) is approximately 47 mmHg. Assuming FiO2 of 0.21: Inspired PO2 = (347 - 47) * 0.21 = 300 * 0.21 = 63 mmHg.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because, without calculation, we cannot assume its correctness. Let's assume it's lower or higher than the correct answer; the actual calculation will prove its incorrectness.
- **Option B:** Similarly, this option would not match the calculated inspired PO2 based on the provided atmospheric pressure and standard physiological parameters.
- **Option D:** This option suggests a much higher PO2 than what would be expected at such a high altitude with significantly reduced atmospheric pressure.

## **Clinical Pearl / High-Yield Fact**
At high altitudes, the decrease in atmospheric pressure results in lower oxygen levels in the air, which can lead to **hypoxia**. The body compensates through **hyperventilation**, increasing ventilation to take in more oxygen. However, this compensation is limited, and individuals may still experience **acute mountain sickness (AMS)**.

## **Correct Answer:** . 63 mmHg